Click on your account email in the upper right area (see screenshot below for exact location)
Your Bing Ads Customer ID will be on the dropdown under your name of the dropdown (see screenshot below for exact location)
Bing Customer ID for Agency Accounts
If you are a Bing advertising agency and have multiple ad accounts, your Bing Customer ID for each customer will be listed in your Bing account manager. You may have access to multiple MCC accounts, in this case, select the master account in the top right, and the customers account number will be listed on the left under their account name.